MINNESOTA GEOLOGY IN CROSS SECTION


by Mark Jirsa

These diagrams present key elements of Minnesota's geology in the form of simplified cross sections that extend from the north-central to the southeastern part of the state. The cross sections show several of the ways that the rocks can be subdivided. The diagrams are schematic and are drawn to illustrate the varied rock types and structural features at depths that can range from a fraction of a kilometer to several kilometers.

sectiona
Section A: Subdivisions by rock and sediment types.

sectionb
Section B: Rock and sediment types subdivided into categories known as Time-Stratigraphic Units. The rocks are assigned to a particular category on the basis of best known relative or absolute age.

sectionc
Section C: Bedrock units showing subdivisions based on both age and geologic characteristics; for example, different types of rocks that formed or were altered during a regional event in the geologic history are grouped together.


Explanation

On the left (north) side of the cross sections are various Archean (about 2.7 billion years old) rock units. The large, patterned (+) ovoid in the left-central part of the section represents the Archean Giants Range batholith, a large granitic intrusion.

To its right are Early Proterozoic (about 1.9-2.2 billion years old) metamorphosed sedimentary rocks.

The bowl-shaped Animikie basin consists of slate, graywacke and iron-formation, the latter shown in red. The gently dipping iron-formation on the basin's left side is the Biwabik Iron Formation of the Mesabi range; the folded unit on the right is the Trommald Iron Formation of the Cuyuna range.

The southern margin of the Early Proterozoic group of rocks is marked by deformation and faulting within structural zones that are remnants of the Penokean mountain building event. For additional information about this area see the Geology of Stearns County

To the right (south) of these faults lie gneissic and granitic rocks of both Archean and Early Proterozoic age. The Archean gneisses of this district are some of the oldest (about 3.6 billion years old) rocks known in North America. Some of these rocks can be seen in outcrop within the Minnesota River Valley.

The gneisses and granitic rocks are on-lapped by Middle Proterozoic volcanic and sedimentary rock of the Keweenawan (1.2-1.0 billion years old) Supergroup--part of the Midcontinent rift system. These strata are best exposed along Minnesota's north shore of Lake Superior.

The nearly flat-lying rock units in the shallow basin on the right side (southern end) of the section are Paleozoic sandstone, shale, and dolostone. They were deposited about 515-365 million years ago when shallow continental seas encroached into Minnesota from the south.

The youngest known pre-glacial, strata in Minnesota are thin units of shale and sandstone--the remnants of broad seas that covered much of the state during the Cretaceous time period, about 100-75 million years ago. Cretaceous rocks are abundant in western and southwestern Minnesota but occur mostly as scattered deposits in the area intersected by this cross section.

Quaternary (about 2 million to 10,000 years old) glacial tills and outwash deposits as much as several hundred feet thick were laid down by continental glaciers that repeatedly advanced across the state from spreading centers in Canada. These deposits form the majority of the land surface and topographic features that we see in Minnesota today.

Minnesota geology